Introduction
The present invention relates to the drying of microspheres, and to the dry expansion of microspheres.
Microspheres are heat expandable thermoplastic polymeric hollow spheres containing a thermally activatable expanding agent. Such materials, the method of their manufacture, and considerable information concerning the properties and uses of microspheres, are all set forth in U.S. Pat. No. 3,615,972, issued to Donald S. Morehouse. Other teachings concerning such materials are found in, for example, U.S. Pat. Nos. 3,864,181; 4,006,273; and 4,044,176.
In following the teachings of the Morehouse patent, the microspheres are made in an aqueous system by the limited coalescence process under pressure, and the resulting product is a "wet cake" of the unexpanded microsphere beads wet with water. The wet cake is typically about 40 to 75 weight percent solids, and because of the wetting agents employed in the formation of the beads, the surface will be wet. Separation of water and beads has not been a simple process.
Many important uses of the microspheres require the removal of the water, to produce dry, free flowing microsphere beads. In many uses, it is also appropriate to pre-expand the beads before use. In unexpanded form the dry microsphere beads typically have a displacement density of about 1.1. Dry, expanded microspheres typically have a density by displacement of less than 0.06 grams/cc, and are highly useful in the production of syntactic foams in a wide variety of polymer matrices. Dry, free-flowing microspheres, and dry, free-flowing, preexpanded microspheres have now achieved a commercial recognition and market demand for such uses and others.
Prior Art
There have been to date few procedures by which dry expanded microspheres have been produced from wet cake. Dry free-flowing unexpanded microsphere beads have not heretofore been available except as a laboratory curiosity, and no commercially practicable procedure for producing such a product has as yet emerged.
In U.S. Pat. No. 4,397,799, pre-expanded, dry, low density microspheres are produced by spray drying. In addition to the inherent expansion of the beads, the spray drying procedure has several disadvantages. First and foremost, the erection of a dedicated spray drying facility is required, representing a very substantial capital investment, and very considerable operating expenses, particularly for skilled labor and utilities costs in heating the drying fluid. In addition, the product is produced entrained in a heated, moving fluid stream, and the requirements of collection, recovery and handling are considerable. In addition, it is not feasible to produce the expanded product at the point of use, because of the size and expense of the facility, so that the spray drying approach largely mandates shipping of large volumes at considerable expense. Not to be neglected is the requirement that the spray drying be conducted under an inert atmosphere, since the usual blowing agents entrained within the microspheres are generally highly flammable, and often explosive. The usual procedure is to employ nitrogen as the spray drying fluid, with a necessary, and highly expensive safety burden on the system. Recovery of the expanded beads from the system is also demanding, not only to avoid product losses, but also because of the dust pollution of the working environment and atmosphere that can result.
Expansion of microsphere beads is attained in the process of U.S. Pat. No. 4,513,106, where the wet cake is injected into a flow stream of steam, which is subsequently quenched in cool water. This produces pre-expansion of the microspheres, but still wet with water, at low solids content. While solids contents as high as fifteen percent have been occasionally been produced, the typical product is more often about 3 to 5 percent solids. The water limits the applications to which the procedure is applicable.